Probiotic Bacteria May Help Treat Depression

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Bacteria in the gut can directly influence the brains of mice and
perhaps humans, scientists find.

This discovery could lead to new ways to control depression,
anxiety and other disorders. And while mice serve as good models
for understanding aspects of the human brain, the findings need
to be replicated in humans.

The scientists detailed their findings online Aug. 29 in the
Proceedings of the National Academy of Sciences.

Probiotic and stress

Researchers have increasingly begun to suspect the gut was
somehow linked with the brain. For instance, bowel disorders seem
linked with stress-related
psychiatric disorders such as anxiety and depression in
people.

To learn more, scientists experimented with mice by feeding them
a broth containing Lactobacillus rhamnosus JB-1. This
species naturally lives in our gut, and scientists are exploring
whether strains of it can be used as "probiotics" to improve our
health. They discovered these rodents displayed significantly
less behavior linked with stress, anxiety and depression than
mice fed plain broth. Bacteria-fed mice also had significantly
lower levels of the stress hormone corticosterone in response to
stressful situations such as mazes.

"By affecting
gut bacteria, you can have very robust and quite
broad-spectrum effects on brain chemistry and behavior,"
researcher John Cryan, a neuroscientist at University College
Cork in Ireland, told LiveScience.

"Without overstating things, this does open up the concept that
we could develop therapies that can treat psychiatric disorders
by targeting the gut," Cryan added. "You could take
a yogurt with a probiotic in it instead of an
antidepressant."

"Now, that would not be an everyday yogurt — I'm not saying you
should go out to the supermarket and try doing this," he
cautioned. "The effect depends on the strain of probiotic you
use. The hope would be, though, that this could have less side
effects than drugs."

The vagus nerve

To figure out the probiotic-stress link, the researchers probed
the brains of the bacteria-fed mice, investigating molecules
called receptors that brain cells use to receive signals. In
particular, they focused on receptors for gamma-aminobutyric
acid, or GABA. This neurotransmitter, the target of the
anti-anxiety drug Valium, is the main chemical used to inhibit
activity in the central nervous system and regulates many
physiological and psychological processes.

The investigators found that one GABA receptor component was
present in higher levels in bacteria-fed mice in parts of the
brain where it is normally lowered during depression. In
addition, several GABA receptor components were reduced in parts
of the brain where they are normally increased in stressed or
anxious animals.

Next, the researchers severed the vagus nerve, which helps alert
the central nervous system to changes in the gastrointestinal
tract. They found the bacteria-induced effects on behavior and
GABA receptors were diminished, suggesting this nerve is the
pathway by which changes in the gut can influence the brain.

Vagal nerve stimulations have been used at times to treat
depression resistant to other therapies, but "that's a surgical
technique," Cryan said. "By targeting
the gut with probiotics, we could indirectly target the vagus
nerve without surgery."

Follow-up studies will investigate whether these bacteria have
this effect alive or dead, to see if it is due to either
something the microbes are equipped with or release; further
studies can also tease out whether the gut can affect other brain
chemicals such as serotonin and dopamine, which have been linked
with mood, among other factors. "Also, we found this in mice —
this is a long way from clinical applications in people," Cryan
said.